Who played the mother in Psycho?

Answer 1

Nobody- the whole point is that Norman Bates's mother was long dead, and that he had preserved her corpse using his taxidermy skills. His psychotic condition made him alternately assume her personality and then his own- when he committed murders, he would dress up in her old clothes believing himself to be her at that time, talk in her voice, and afterwards snap back into his own personality. His own character and his delusional 'mother' character would each blame the other for the killing.

Thus the 'mother' character was played by Anthony Perkins all along, wearing old ladie's clothes. The corpse discovered in the fruit cellar at the end was just a wax dummy, it wasn't a real person in make-up.